1. Field of the Invention
The present invention relates to an optical scanning device and an image forming apparatus using the optical scanning device, and, more particularly to a setting structure for optical elements.
2. Description of the Related Art
In an image forming apparatus like a copying machine, a printer, or a facsimile apparatus, an optical scanning device using a light beam like a laser beam may be used as one of devices that form an electrostatic latent image on a photosensitive member used as a latent image bearing member (see, for example, Japanese Patent Application Laid-Open No H6-148553 (paragraph 0002) and Japanese Patent Application Laid-Open No. H10-232360 (paragraph 0004)).
On the other hand, there is an apparatus using an electrophotographic technology like a digital copying machine or a laser printer that are a kind of an image forming apparatus. The apparatus uses light spot to scan a photosensitive member in a direction perpendicular to the moving direction of the photosensitive member with an optical scanning device to write a latent image on the photosensitive member, develops the latent image using a toner, transfers a toner image on paper, heats the toner with a fixing device, and forms an image on the paper. There is also a tandem color image forming apparatus or the like that performs image forming processes for a plurality of colors in parallel using a plurality of photosensitive members and developing devices to form a color image at high speed (see, for example, Japanese Patent Application Laid-Open No. 2003-248186 (paragraphs 0005 and 0041).
An optical scanning device used in such an image forming apparatus uses a semiconductor laser as a light source, shapes a light beam emitted from the semiconductor laser with a first imaging element like a collimator lens or a cylindrical lens and, then, deflects the light beam with a deflector like a polygon scanner, focuses the light beam on an image surface as a light spot via a second imaging element like an fθ lens or an optical face tangle error correction lens of the deflector, and uses the light beam to scan the image surface. The optical scanning device is often reduced in size and an arrangement position of the optical scanning device in the image forming apparatus is often optimized by inserting a mirror in an optical path from the light source to the image surface to bend the optical path.
As the optical scanning device in the image forming apparatus of the tandem system, there is an optical scanning device that uses a light beam to scan four photosensitive members, respectively. In such an optical scanning device, since only one expensive deflector has to be used, there is an advantage that it is possible to manufacture the optical scanning device at low cost (see, for example, Japanese Patent Application Laid-Open No. 2003-248186 (paragraphs 0005 and 0041)).
In an optical writing device disclosed in Japanese Patent Application Laid-Open No. 2003-248186, optical elements like a lens and a mirror, a deflector, and the like used for the optical writing device are set in an optical box forming a closed space to prevent pollution of an optical surface due to dust from the outside.
In the optical scanning device including the optical elements and the deflector located in the optical box having the closed space, vibration may be caused and transmitted to the optical box when a rotary polygon mirror used as the deflector is driven to rotate at high speed by a motor. The vibration of the optical box induces vibration in support sections of respective optical elements. As a result, positions and postures of the optical elements positioned in predetermined positions in the optical box fluctuate. It is likely that this causes deterioration in an output image because of fluctuation in a position and a shape of a light spot focused on an image surface. In particular, in the constitution disclosed in Japanese Patent Application Laid-Open No. 2003-248186, that is, the constitution in which a light beam is changed to four light beams by one deflector and guided to optical paths to respective imaging surfaces to realize a reduction in cost, it is necessary to store all the optical elements in the optical box. Thus, a size of the optical box is increased.
Therefore, since a span between frames used in supporting the optical box in the image forming apparatus is extended, it may be difficult to secure rigidity against vibration. Moreover, in such an optical scanning device, the elements significantly affecting optical characteristics such as the deflector and the fθ lens are arranged in the center of the optical box that is most easily vibrated, that is, a position where largest bending deformation occurs among support spans in the support frames. Thus, the vibration of the optical box tends to affect an output image.
In Japanese Patent Application Laid-Open No. 2003-248186, rigidity of the optical box is improved by providing reinforcing ribs to prevent such a deficiency from occurring. However, when the reinforcing ribs are provided among the spans of the support frames, the reinforcing ribs are also located near the deflector. Therefore, to prevent a light beam from the deflector from being blocked by the reinforcing ribs, a part of the reinforcing ribs located near the deflector are removed or notches are provided in the reinforcing ribs. Such a constitution leads to lack of portions where rigidity against vibration is improved near the deflector. This limits an effect of the reinforcing ribs against vibration.